WDM channel monitoring system and method

ABSTRACT

The system and method of the present invention is operative to monitor the performance of an optical communications channel. An optical splitter splits a wavelength division multiplexed (WDM) optical communications signal into a low power WDM signal onto a back-up path, where a tunable filter sweeps the optical communications channels, while a monitoring circuit monitors the optical communications channels for performance. Optical power can be stored and subsequently displayed, providing an optical spectrum analysis of the signal.

FIELD OF THE INVENTION

[0001] This invention relates to the field of optical communicationsystems, and more particularly, this invention relates to an opticalcommunication system using wavelength division multiplexed (WDM) opticalcommunication signals and having back-up receiver capability.

BACKGROUND OF THE INVENTION

[0002] Wavelength division multiplexing (WDM) is commonly used inoptical communication systems for increasing the bandwidth of a fiberoptic telecommunications link, without increasing the speed ofassociated electronics. In many prior art optical communicationtechniques, the bandwidth of a single channel (or wavelength fiber optictelecommunication link) has been limited primarily by the high-speedelectronics required at the transmitter and receiver. By usingwavelength division multiplexing at a telecommunications systemreceiver, the optical channels that receive the optical communicationsignals are separated, or demultiplexed, and sent to individualreceivers, which vary in their rate of data receipt. One example of areceiver is a 2.488 Gb/S receiver.

[0003] The number of individual receivers used in the opticalcommunications system can vary. These communication receivers connectinto a back plane of existing telecommunications equipment. For example,a telecommunications rack could include one or more receivers, such as 8or 16 receivers, each mounted on a board within the telecommunicationsrack. When optical components fail, it is necessary to determine thechannel that is being used by the failed optical component or particularreceiver.

[0004] In the past, telecommunication links have rerouted signals on theelectrical switching level when any optical components failed, thusloading another path onto the network. It would be more advantageous tore-route an optical communication signal on a particular wavelengthchannel at the receiver terminal, in the case of a receiver failure orother optical component failure, and not consume network bandwidth as inprior art techniques. This would allow receiver maintenance at any timewithout increasing downtime or network re-routing.

[0005] It would also be desirable to monitor a channel and allowcontinuous sweeping of the optical communications channels. For example,if a channel showed any signs of weakening or failure, it would beadvantageous to identify the source of the problem so that correctivemeasures could be sought. Thus, there is a need for greater channelmonitoring capability. Although there are some channel monitoringdevices that use single mode fiber, such as one commercially availablesystem manufactured under the trade designation “Spectra SPAN,” it hasno capability as a back-up signal receiver.

SUMMARY OF THE INVENTION

[0006] The present invention is advantageous and allows the re-routingof optical communication signals at the receiver terminal, in case ofreceiver failure or other optical component failure. The system alsodoes not consume network bandwidth as in past practices, where signalshave been re-routed on the electrical switching level when opticalcomponents failed. Thus, in the present invention, another path is notloaded onto the network and bandwidth is not consumed. The presentinvention also allows receiver maintenance at any time, without downtime or network re-routing.

[0007] The present invention can also function as a channel monitor,allowing continuous sweeping of optical communication channels forquality and performance. When a channel shows signs of weakening orfailure, identification of the source of the problem can be triggered,and corrective measures sought. If any one of the dedicatedtelecommunications system receivers fail on any given wavelength, theback-up receiver system of the present invention can be tuned to thatparticular wavelength and take over the link, while repairs are beingconducted.

[0008] The present invention can also be used as a tracking filter forsystems that use a tunable laser for laser transmitters that fail. Thereceiver can track to a new wavelength location where a tunabletransmitter has been positioned to account for a failing, or a failedlaser transmitter. The present invention can also be used as a tunablereceiver for systems/locations requiring tunability, such as add/dropnodes on a fiber.

[0009] In accordance with the present invention, the system monitors theperformance of an optical communications channel and includes an opticalsplitter positioned along an optical communications path for receiving awavelength division multiplexed (WDM) optical communications signal onthe optical communications path. This signal is split into a low powerWDM signal onto a back-up path where a tunable filter receives the lowpower WDM optical signal and sweeps the optical communications channels.A monitoring circuit is operatively connected to the tunable filter andmonitors the optical communications channels for performance. Thetunable filter is swept and the optical power is stored and subsequentlydisplayed, providing an optical spectrum analysis of the signal. Theoptical amplifier can receive the low power WDM signal and amplify sameafter splitting from the optical communications signal.

[0010] In another aspect of the present invention, the amplifierincludes an injection laser diode and a current source control loopcircuit connected to the injection laser diode that establishes a fixedcurrent through the injection laser diode. A voltage switcher circuit isconnected to the injection laser diode and current source control loopcircuit. The tunable filter can comprise a Fabry Perot filter. Thecontroller can be operatively connected to the tunable filter in acontroller feedback path for controlling the selection of desiredwavelengths corresponding to the optical communications channels.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Other objects, features and advantages of the present inventionwill become apparent from the detailed description of the inventionwhich follows, when considered in light of the accompanying drawings inwhich:

[0012]FIG. 1 is a block diagram of a wavelength division multiplexedoptical communications system having a plurality of fixed receivers anda tunable optical receiver of the present invention positioned on theback-up path, which are connected to the back plane of existingtelecommunications equipment.

[0013]FIG. 2 is another block diagram showing add/drop nodes, where atunable receiver, processing equipment, and tunable transmitter areused.

[0014]FIG. 3 is another block diagram showing an example of a wavelengthdivision multiplexed optical communications system having the opticallyamplified back-up receiver of the present invention.

[0015]FIG. 4 is another block diagram similar to FIG. 3, but showing ingreater detail the optically amplified back-up receiver of the presentinvention.

[0016]FIG. 5 is an enlarged block diagram of the tunable filter of thepresent invention having optical channel monitoring capability with aspectrum analyzer.

[0017]FIG. 6 is a block diagram of a low power laser diode driver usedas part of the amplifier section of the optically amplified back-upreceiver of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention will now be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0019] The present invention is advantageous and allows the re-routingof optical communication signals at the receiver terminal, in case ofreceiver failure or other optical component failure. The system alsodoes not consume network bandwidth as in past practices, where signalshave been re-routed on the electrical switching level when opticalcomponents failed. Thus, in the present invention, another path is notloaded onto the network and bandwidth is not consumed. The presentinvention also allows receiver maintenance at any time, without downtime or network re-routing.

[0020] The present invention can also function as a channel monitor,allowing continuous sweeping of optical communication channels forquality and performance. When a channel shows signs of weakening orfailure, identification of the source of the problem can be triggered,and corrective measures sought. If any one of the dedicatedtelecommunications system receivers fail on any given wavelength, theback-up receiver system of the present invention can be tuned to thatparticular wavelength and take over the link, while repairs are beingconducted.

[0021] The present invention can also be used as a tracking filter forsystems that use a tunable laser for laser transmitters that fail. Thereceiver can track to a new wavelength location where a tunabletransmitter has been positioned to account for a failing, or a failedlaser transmitter. The present invention can also be used as a tunablereceiver for systems/locations requiring tunability, such as add/dropnodes on a fiber.

[0022]FIG. 1 illustrates an optical communication system at 10, where awavelength division multiplexed (WDM) optical communications signal 12is transmitted along optical communications path 13. The opticalcommunications signal 12 passes through an optical splitter 14,positioned along the path 13, which splits off about 5%, as anon-limiting example, of the light power comprising the opticalcommunications signal and as a full spectrum of that signal into aback-up path 15. About 95% of the wavelength division multiplexedoptical communication signal passing along the optical communicationspath continues into a WDM circuit 16, which includes a demultiplexer fordividing the WDM optical communications signal into separate opticalcommunication signals of different wavelengths λ1 through λn, and intorespective fixed (dedicated) receivers 18, such as the illustrated fixedreceiver 1, fixed receiver 2, . . . fixed receiver n. The fixedreceivers 18 connect into the back plane 20 of existingtelecommunications equipment, as known to those skilled in the art.

[0023] The optical signal 22 is formed from the split-off portion of thelight and comprises about 5% of the original power of the WDM opticalcommunications signal 12. It is a low power WDM optical signal havingthe full spectrum of the light from the original WDM opticalcommunications signal 12. The tunable, optically amplified back-upreceiver 24 of the present invention receives the optical signal 22 andselects an optical signal of desired wavelength for the appropriatechannel to be backed-up (λ1 through λn), and converts this opticalsignal of desired wavelength into an electrical communications signal tobe fed into the back plane 20. The receiver 24 of the present inventionoperates as a back-up receiver in case one of the fixed receivers 18 isinoperable, or the optical components that carry an optical signal of aparticular wavelength are inoperable.

[0024]FIG. 2 illustrates how the tunable, optically amplified back-upreceiver 24 of the present invention can be used in a system or locationrequiring tunability, such as for respective add/drop nodes 26,28 on atelecommunications fiber. The tunable receiver 24 is operativelyconnected to telecommunications processing equipment 32 and a tunabletransmitter 34. The optical signal of selected wavelength can be droppedand received in the tunable receiver 24. It is converted by the receiver24 into the appropriate electrical communications signal, which is thenprocessed by appropriate signal processing circuitry, amplifiercircuitry, regeneration circuitry and other circuitry known to thoseskilled in the art. Once processed, the electrical communications signalis passed to the tunable transmitter 34, which converts the electricalcommunications signal that had been processed into an optical signal. Itis then added to the main optical communications signal 12 passing alongthe main optical communications path 13.

[0025]FIG. 3 illustrates a high level block diagram of the opticalcommunications system 10 where optical communications signals 35 ofabout 1550 nanometers are wavelength division multiplexed 36 intooptical communications signal 12 at about 155 Mb/S to about 4 Gb/S in ahigh bandwidth data distribution system 37, including appropriatein-line, erbium doped fiber amplifiers 38 acting as optical repeaters.The amplified optical communications signal 12 is passed to thededicated optical receivers 18 along the main optical communicationspath 13.

[0026] The optical splitter 14 forms a node that allows the fullspectrum of the wavelength division multiplexed optical communicationssignal to be split off (about 5% of its power) and passed into back-uppath 15 as an optical signal 22 and to the tunable mini/low poweroptically amplified back-up receiver 24 of the present invention.

[0027] The tunable optically amplified back-up receiver 24 of thepresent invention includes an erbium doped fiber amplifier 44 (EDFA)acting as a preamplifier. This permits amplification of the low poweroptical signal before passing into the tunable bandpass filter 46, whichselects one of the desired wavelengths, λ1 through λn. A photodetector,which in the present embodiment is a PIN diode 48, but also can be anAvalanche Photo Diode (APD), converts the amplified and optical signalof desired wavelength into an electrical communications signal andpasses that electrical communications signal into a low-noise electricalamplifier 50 and into the clock and data recovery circuit 52.

[0028]FIG. 4 illustrates greater details of the tunable opticallyamplified back-up receiver 24 of the present invention, and illustratingthree main sections as an amplifier section 54, having the erbium-dopedfiber amplifier (EDFA) 44 as shown in FIG. 3, a tunable filter section56, and the receiver section 58 operable as a detector used at differentwavelengths. The detector electronics is selected to support typicaldata rates, including 2.5 and 10.0 Gb/S.

[0029] Although the ranges of data and number of used channels are setforth as non-limiting examples, it should be understood that the presentinvention is advantageously used with different wavelengths anddifferent number of channels. As illustrated, the WDM opticalcommunications signal, such as 2.5 Gb/S WDM signal input, passes into a1550/980 WDM input circuit 58 a that is operable with a Fiber BraggGrating Stabilized Pump Laser Diode circuit 59 and a low power laserdiode driver circuit 60.

[0030] Although different laser diode drivers can be used in accordancewith the present invention, in one aspect of the present invention, thelow power laser diode driver is illustrated in FIG. 6, and can be usedwith the tunable receiver of the present invention. This low power laserdriver circuit 60 can be used for driving the optical preamplifier andreceiver assembly shown in FIG. 4.

[0031] A five volt supply voltage input is standard with many electroniccircuits. The laser driver circuit 60 includes an injection laser diode62 that is, in one aspect of the present invention, a high quantumefficiency injection laser diode (HQEILD). A current source control loopcircuit 64 is connected to the injection laser diode 62 and establishesa fixed current through the injection laser diode. This current sourcecontrol loop circuit 64 has a voltage switcher circuit chip 66 connectedto the injection laser diode, within the current source control loopcircuit, and is adapted to receive the fixed supply voltage of fivevolts and convert inductively the supply voltage down to a forwardvoltage, to bias the laser injection diode and produce an optical outputhaving minimized power losses.

[0032] This voltage switcher circuit chip 66 is monolithically formed asa single circuit chip, and is used as a high efficiency voltageconverter as shown in FIG. 6.

[0033] The current source control loop circuit 64 includes the highefficiency current source 70, acting as a low noise current source andthe current control circuit 72. These circuits are all contained withinone housing, and in one aspect, on a printed circuit card assembly 74that includes the receiver components, including the preamplifier,tunable bandpass filter circuit and optical-to-electrical conversioncircuit.

[0034] The schematic circuit diagram shows various power and voltage, aswell as current parameters. In this non-limiting example, at 260milliwatts and at five volts DC, there is a 35 decibel optical gain,with one channel as a design goal. There could be a 266 milliwatt DC foreight channels, and 220 milliwatts DC achieved. The Bragg grating 73 isoperatively connected to the injection laser diode 62, and is operativeby principles known to those skilled in the art. The Bragg grating 73 isconfigured for receiving the optical output and stabilizing the opticalwavelength.

[0035] As shown in FIG. 4, an ASE Reduction Stage circuit 80 works inconjunction with an isolator circuit 82 using amplification techniquesknown to those skilled in the art. The tunable filter section 56includes the tunable filter 46, which in one aspect of the presentinvention, is a fiber Fabry Perot tunable filter 84. A 1:99 coupler 88,as a non-limiting example, allows a portion of the optical signal to beconverted by a photodetector to an electrical current, and pass into afeedback control circuit 87, including an analog/digital converter 88, alow power controller 90, which is operative with a controller interface92 and associated electronics, and digital/analog converter 94, forconverting digitally processed control signals back to analog controlsignals and selectively tuning the fiber Fabry Perot tunable filter.This circuitry also allows an optical spectrum to be detected andstored.

[0036] The optical communications signal, once tuned to the desiredwavelength and frequency, passes into the receiver section 58 thatincludes an optical-to-electrical conversion circuit having thedetector, i..e, the PIN photodiode 48, followed by the low noiseelectrical amplifier 50, which in one aspect of the invention, is apreferred transimpedance amplifier and amplifies the convertedelectrical communication signal received from PIN photodiode 48. Anelectronic limiter circuit 96 receives the electrical communicationssignal and works in conjunction with a clock and data recovery circuit52. This circuit allows data recovery and reshaping of electricalcommunication signals. A clock recovery circuit portion of circuit 52allows recovery of clock signals and retiming of electricalcommunication signals by techniques known to those skilled in the art.

[0037] The data is output to the back plane 20 as shown in FIG. 1. Inthe embodiment shown in FIG. 2, the signal is sent to the processingequipment 32, and tunable transmitter 34, which then passes the signalback onto the main optical communication path 13.

[0038] In one non-limiting example of the present invention, theamplifier section 54 has about 230 mW with commercial off the shelfcomponents (COTS) of about 2.0 watts, followed by the tunable filtersection 56 operable at about 50 mW and COTS of about 6 W, and thereceiver section 58 of about 680 mW and COTS of 1.5 W for a 2.5 Gb/Sdata rate.

[0039] The optical sensitivity at 2.5 Gb/S can be less than about −40dBm at 1×10⁻¹⁰ BER (bit error rate) with a total one channel powerconsumption of about 960 mW. For a non-limiting example of eightchannels, it is possible to use a fixed λ demultiplexer providing atotal power consumption of about 5.7 W, corresponding to 710 mW/channel.

[0040] Referring now to FIG. 5, there is illustrated the tunable filterthat has been modified to have channel monitoring or optical spectrumanalysis capability in accordance with another aspect of the presentinvention. Between the analog/digital conversion circuit 88 and the lowpower controller 90, an optical channel monitoring circuit 100 isconnected. The circuit 100 can include a spectrum analyzer, power meteror other associated electronic equipment for monitoring the channel.Thus, it is possible to select various wavelengths to monitor theoperation of the particular channel and determine if there are errors indata transmission or other selected aspects. In this aspect of theinvention, the tunable filter can be swept and the optical power storedin a processor memory, controller or other means known to those skilledin the art. This data can be processed and subsequently displayed,providing an optical spectrum analysis of the signal. The system canmonitor averaged power and supervisory communications data. It also canperform an optical spectrum analysis of the signals.

[0041] It is also possible to use the optically amplified back-upreceiver as a tracking filter for systems that use the tunable laser forlaser transmitters that fail. The receiver can track to a new wavelengthlocation where the tunable transmitter has been positioned to accountfor failing or failed laser transmitter.

[0042] This application is related to copending patent applicationentitled, “OPTICALLY AMPLIFIED BACK-UP RECEIVER,” which is filed on thesame date and by the same assignee and inventors, the disclosure whichis hereby incorporated by reference.

[0043] Many modifications and other embodiments of the invention willcome to the mind of one skilled in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is to be understood that the invention is not tobe limited to the specific embodiments disclosed, and that themodifications and embodiments are intended to be included within thescope of the dependent claims.

That which is claimed is:
 1. A system for monitoring the performance ofoptical communications channels comprising: an optical splitterpositioned along an optical communications path for receiving awavelength division multiplexed (WDM) optical communications signal onthe optical communications path and splitting a portion of the WDMoptical communications signal into a low power WDM signal onto a back-uppath; a tunable filter that receives the low power WDM optical signaland sweeps the optical communications channels; and a monitoring circuitoperatively connected to said tunable filter for monitoring the opticalcommunications channels for performance.
 2. A system according to claim1, wherein said tunable filter is swept and the optical power is stored,and subsequently displayed, providing an optical spectrum analysis ofthe signal.
 3. A system according to claim 1, and further comprising anoptical amplifier for receiving the low power WDM signal and amplifyingsame after splitting from the optical communications signal.
 4. A systemaccording to claim 3, wherein said amplifier comprises: an injectionlaser diode; a current source control loop circuit connected to saidinjection laser diode that establishes a fixed current through theinjection laser diode; and a voltage switcher circuit connected to saidinjection laser diode and current source control loop circuit.
 5. Asystem according to claim 1, wherein the tunable filter furthercomprises a Fabry Perot filter.
 6. A system according to claim 1, andfurther comprising a controller operatively connected to said tunablefilter in a controller feedback path for controlling the selection ofdesired wavelengths corresponding to the optical communicationschannels.
 7. A system according to claim 6, wherein said monitoringcircuit is operatively connected to said controller within saidcontroller feedback path.
 8. A system according to claim 7, wherein saidcontroller feedback path further comprises analog/digital converter andoptical coupler operatively connected to said tunable filter andcontroller and a digital/analog converter operatively connected to saidcontroller and tunable filter, wherein said monitoring circuit isoperatively connected to said controller and said analog/digitalconverter.
 9. A system for monitoring the performance of opticalcommunications channels comprising: an optical splitter positioned alongan optical communications path for receiving a wavelength divisionmultiplexed (WDM) optical communications signal on the opticalcommunications path and splitting a portion of the WDM opticalcommunications signal into a low power WDM signal onto a back-up path; atunable filter that receives the low power WDM optical signal and sweepsthe optical communications channels; a monitoring circuit operativelyconnected to said tunable filter for monitoring the opticalcommunications channels for performance; and a detector circuit forreceiving the optical signal of desired wavelength and converting theoptical signal into an electrical signal to monitor averaged power,supervisory communications data and/or perform an optical spectrumanalysis of the signals.
 10. A system according to claim 9, wherein saiddetector circuit comprises a PIN diode.
 11. A system according to claim9, wherein said detector circuit comprises an Avalanche Photodiode. 12.A system according to claim 9, wherein said detector circuit comprisesan amplifier circuit.
 13. A system according to claim 9, wherein saiddetector circuit comprises a limiter circuit for reshaping the detectedoptical signal.
 14. A system according to claim 9, wherein said detectorcircuit comprises a data decision circuit and clock recovery circuit.15. A system according to claim 9, and further comprising an opticalamplifier for receiving the low power WDM signal and amplifying sameafter splitting from the optical communications signal.
 16. A systemaccording to claim 9, wherein said amplifier comprises: an injectionlaser diode; a current source control loop circuit connected to saidinjection laser diode that establishes a fixed current through theinjection laser diode; and a voltage switcher circuit connected to saidinjection laser diode and current source control loop circuit.
 17. Asystem according to claim 9, wherein the tunable filter furthercomprises a Fabry Perot filter.
 18. A system according to claim 9, andfurther comprising a controller operatively connected to said tunablefilter in a controller feedback path for controlling the selection ofdesired wavelengths corresponding to the optical communicationschannels.
 19. A system according to claim 18, wherein said monitoringcircuit is operatively connected to said controller within saidcontroller feedback path.
 20. A system according to claim 19, whereinsaid controller feedback path further comprises analog/digital converterand optical coupler operatively connected to said tunable filter andcontroller and a digital/analog converter operatively connected to saidcontroller and tunable filter, wherein said monitoring circuit isoperatively connected to said controller and said analog/digitalconverter.
 21. A method of monitoring the performance of opticalcommunications channels of a wavelength division multiplexed (WDM)optical communications signal comprising the steps of: splitting off apercentage of the power from the WDM optical communications signal as anoptical signal into a back-up path; sweeping the optical communicationschannels by filtering the optical signal within a tunable filter andselecting desired wavelengths corresponding to respective opticalcommunications channels; and monitoring the optical communicationschannels for performance.
 22. A method according to claim 21, whereinthe percentage of split off from the WDM optical communications signalis about five percent.
 23. A method according to claim 21, and furthercomprising the step amplifying the optical signal after splitting offfrom the optical communications signal and before sweeping within thetunable filter.
 24. A method according to claim 21, and furthercomprising the step of amplifying the optical signal in an amplifiercircuit comprising: an injection laser diode; a current source controlloop circuit connected to said injection laser diode that establishes afixed current through the injection laser diode; and a voltage switchercircuit connected to the injection laser diode and current sourcecontrol loop circuit.
 25. A method according to claim 21, and furthercomprising the step of sweeping the optical signal within a Fabry Perottunable filter.
 26. A method according to claim 21, and furthercomprising the step of controlling the selecting of desired wavelengthswithin the tunable filter via a controller operatively connected to thetunable filter in a controller feedback path.
 27. A method according toclaim 26, and further comprising the step of monitoring the performanceof the optical channels via an optical channel monitoring apparatusoperatively connected to said controller.
 28. A method according toclaim 27, wherein the step of monitoring comprises the steps ofmonitoring within a spectrum analyzer.
 29. A method of monitoring theperformance of optical communications channels of a wavelength opticalcommunications signal comprising the steps of: splitting off apercentage of the power from the WDM optical communications signal as anoptical signal into a back-up path; sweeping the optical communicationschannels by filtering the optical signal within a tunable filters andselecting desired wavelengths corresponding to respective opticalcommunications channels; monitoring the optical communications channelsfor performance; and detecting and converting the optical signal ofselected wavelength into an electrical communications signal.
 30. Amethod according to claim 29, wherein the percentage of split off fromthe WDM optical communications signal is about five percent.
 31. Amethod according to claim 29, and further comprising the step ofamplifying the detected optical signal.
 32. A method according to claim29, and further comprising the step of reshaping the detected opticalsignal within a limiter circuit.
 33. A method according to claim 29, andfurther comprising the step amplifying the optical signal aftersplitting off from the optical communications signal and before sweepingwithin the tunable filter.
 34. A method according to claim 29, andfurther comprising the step of amplifying the optical signal in anamplifier circuit comprising: an injection laser diode; a current sourcecontrol loop circuit connected to said injection laser diode thatestablishes a fixed current through the injection laser diode; and avoltage switcher circuit connected to the injection laser diode andcurrent source control loop circuit.
 35. A method according to claim 29,and further comprising the step of sweeping the optical signal within aFabry Perot tunable filter.
 36. A method according to claim 29, andfurther comprising the step of controlling the selecting of desiredwavelengths within the tunable filter via a controller operativelyconnected to the tunable filter in a controller feedback path.
 37. Amethod according to claim 36, and further comprising the step ofmonitoring the performance of optical channels via optical channelmonitoring apparatus operatively connected to said controller.
 38. Amethod according to claim 37, wherein the step of monitoring comprisesthe steps of monitoring within a spectrum analyzer.